Various techniques for rendering polymeric films opaque have been developed in the past. Each of these techniques seek to optimize optical opacity in its own way. For example, opaque films are most conventionally prepared by adding a pigment which acts as an opacifying agent to a solution of a film forming material which would otherwise be colorless or transparent when cast in a film. As will be more fully explained hereinafter the amount and size of the pigment particles generally are felt to be the criteria for optimum opacity.
Optical opacity, for example, the hiding power of a paint film, is achieved either by absorption of the incident light or by scattering of the incident light, or a combination of these two. Thus, black is opaque because it absorbs the light incident on it and white is opaque because it back scatters the incident light. Light is either absorbed or scattered before it can reach the substrate. The ideal white pigment then is one which has zero absorption and maximum scattering.
Absorption depends primarily on the electronic structure of the molecule, as well as on the pigment particle size relative to the wave length of light. Scattering depends on the relative refractive indices of pigment and vehicle as well as on the particle size of the pigment relative to the wave length of incident light.
One simple description of the relation of the scattering and absorption to the resulting reflectance is that of Kubelka and Munk. At complete hiding, the following equation applies: ##EQU1## wherein R.infin.is reflectance of a film so thick that a further increase in thickness does not change the reflectance, K is the absorption coefficient and S is the Kubelka-Munk scattering coefficient. No account is taken of the surface reflectances, and the equation applies only to internal reflectance.
The fractions contributed by more than one pigment in a system are additives as shown by the following equation: ##EQU2## wherein C.sub.1, C.sub.2 and C.sub.3 refer to the concentrations of pigments 1, 2, 3, etc.
When hiding is incomplete, the following equation applies: ##EQU3## where R is the resulting internal reflectance, Rg is the reflectance of the substrate, a is equal of (S + K)/S, b = (a.sup.2 - 1).sup.1/2, S is the scattering coefficient, X is the thickness of the film in mils, and ctgh refers to hyperbolic cotangent.
The Kubelka-Munk scattering coefficient may be computed from the following equation: ##EQU4## where Ar ctgh refers to the inverse hyperbolic cotangent, Ro is the reflectance over a black substrate, of 0% reflectance, a may be found from the relation, and b is determined as above. In this equation, R equals reflectance over a white substrate and Rg is reflectance of the substrate which is coated; or a may be found from the following equation: EQU a = 1/2 [1/R.infin. + R.infin.]
k may be found from the equation K = S(a-1).
The Kubelka-Munk analysis is discussed in further detail by D. B. Judd, in "Color in Business, Science and Industry", John Wiley and Sons, New York, 1952, ppg. 314 - 338; and by D. B. Judd and G. Wyszecki in "Color in Business, Science and Industry", 2nd Edition, John Wiley and Sons, New York, 1963, ppg. 387 - 413, the disclosures of which are incorporated herein by reference.
Various other techniques have been developed in the art for preparing opaque films which rely for opacity upon the presence of a large number of voids in the films. Such may be prepared for example, by depositing a film from an emulsion, e.g., either an oil-in-water or a water-in-oil emulsion. When a water-in-oil emulsion is used, i.e., one in which minute droplets of water are dispersed in a continuous phase of a film forming material -- the emulsion is deposited as a coating and the organic solvent which comprised the continuous phase of the emulsion is evaporated therefrom. This causes gelation of the film forming material and entrapment of the dispersed water droplets. The water is then evaporated leaving microscopic voids throughout the film structure.
When the oil-in-water emulsion is used, the mechanism for forming the film is similar to that described above. A film forming material is dissolved in water. Thereafter, an organic liquid which is a non-solvent for the film former and which is not miscible with water is emulsified in the aqueous phase. The emulsion is formed into a thin layer and the water is evaporated causing the film forming material to gel and entrap minute droplets of the organic liquid. This liquid is then evaporated to cause minute voids in the film structure.
Another technique for obtaining porous, opaque, non-pigmented films is by preparing an aqueous dispersion of a film forming polymer containing a water soluble organic solvent in an amount which is insufficient to dissolve the polymer. A film is then formed from this aqueous dispersion and water is evaporated causing entrapment of minute droplets of the organic solvent in the polymer. The film obtained is then washed to dissolve the entrapped minute droplets of solvent and the film is dried.
Still another technique for obtaining porous, opaque, non-pgimented films is set forth in U.S. Pat. No. 2,961,334. Basically, this process contemplates adding a polymeric material to a liquid solvent to thereby form either a solution or a quasi-solution (i.e., as by peptizing). To this continuous phase is added a liquid which has a higher boiling point than the liquid solvent and which is a non-solvent for the film forming polymeric materials. The resulting emulsion is then applied to a substrate whereupon an opaque film is formed after first evaporating the water and then the non-solvent.
Various techniques have also been developed to modify latex compositions by the addition of a liquid non-solvent for the polymeric material of the latex. One such technique is disclosed in U.S. Pat. No. 3,092,601. This patent discloses a unique method for preparing selfinduced three-dimensional patterns from coating compositions containing a polyvinyl acetate latex, a pattern forming agent (which is a non-solvent for the polymeric material) and various additives. In addition, there may be added a small amount of a pigment or non-leafing metallic powder. Although it is disclosed that the polyvinyl acetate may be modified by copolymerization with up to 20% of another vinyl monomer, or plasticized with a suitable plasticizer, the compositions of the disclosed invention should always be those which do not coalesce well at room temperature in order to obtain the desired self-induced three-dimensional patterns. Therefore, the pattern forming agent which is usually a non-solvent for the polyvinyl acetate polymer acts only as a pattern forming agent and not as a void forming agent which would produce opaque films since the polymeric material would not coalesce enough to entrap sufficient amounts of non-solvent to enable the resulting coating to become opaque upon the later removal of the non-solvent.
Another technique which was recently disclosed in U.S. Pat. No. 3,445,272, relates to the preparation of porous elastomeric coatings from a suspension of an latex of elastomeric polymers containing a waterimmisicble liquid which is a non-solvent for the elastomeric polymer. The composition is applied as a coating and the water and non-solvent are evaporated to leave behind small open cell pores in the resulting coating. While the open cell porous coatings have significant utility when used for shoe uppers, battery separators and the like, it is generally not desirable to employ a highly permeable coating as a protective paint.
Although the above described techniques have proved useful in producing coatings or films which accomplish certain results, no techniques have been disclosed for obtaining a paint a non-porous, microcellular coating which is continuous and opaque in the absence of an opacifying pigment.
Furthermore, the process described above which starts with an aqueous dispersion (i.e., a latex), rather than a solution or quasi-solution of the polymer uses a water-soluble polymer solvent for the pore forming ingredient. This polymer solvent must utimately be washed away leaving an open celled structure. The problems attendant with such a solvent process are well known in the art. For example, the process is limited to the use of polymer solvents which are also water soluble. This condition removes a degree of flexibility from the operation. The washing steps, as another example, add an expense to the process. Further difficulties arise in the formation of open cells since such cells result in high permeability of the final film. Although convenient for some purposes, high permeability is undesirable for many film applications especially in the area of water-repellant and sealing paints.
In summarizing the above-described process for forming opaque films, it may be stated that those processes which contemplate the formation of closed cells in a film generally use the opacifying technique of evaporation to remove the discontinuous phase liquid from the film to thereby prevent rupture of the cells and maintain their closed integrity while at the same time rendering the film opaque. Preferably, the discontinuous phase liquid used is one which will permeate readily through the polymer matrix of the film so that evaporation may be achieved easily and economically. In many processes which envision the eventual formation of open cells or voids, a washing step must be used to wash-out or extract the discontinuous phase liquid from a film and thereby opacify it.
Many of the above-described processes assume the use of good film formers or soluble film formers in order to obtain their desired results. Thus, in this respect, all of the processes are relatively inflexible in their application since many desirable polymers which are not good film formers or which are not soluble film formers at normal temperatures are thereby eliminated from use.
Each polymer has its own "glass transition temperature" (Tg); this term is well known in the art and is generally used to define or describe a temperature above which the polymer has acquired sufficient thermal energy for molecular rotational motion or considerable torsional oscillation to occur about the majority of bonds in the main chain. This term is also used to define a "minimum film forming temperature" above which the polymer has enough internal energy and flow to form a film. In effect, then, the term "glass transition temperature" or "minimum film forming temperature" describes a type of internal melting point for a polymer, not a phase change, at and about which the polymer preserves the outward appearance of a solid but at the same time behaves much like a liquid in its ability to undergo plastic flow and elastic deformation. For the purposes of this invention, the term "glass transition temperature" may be used interchangably with and defined as the "minimum film forming temperature" of a polymer. Although theoretically this temperature is probably an exact point, in practice this point actually turns out to be a small temperature range due to the inability to achieve ideal equilibrium conditions.
For this definition, it is seen that at any given temperature T, a particular polymer and thus its latex may be either a good film former, nonfilm former, or marginal film former depending upon its Tg point (i.e., minimum film forming temperature). For example, if T is taken as room temperature (68.degree. - 75.degree.F.), then any polymer having a Tg substantially greater than room temperature (for example, 90.degree.F.) will be a non-film former at temperature T, while any polymer with a Tg substantially below room temperature (for example, 66.degree.F.) will be a good film former at temperature T. Between these two extremes will be polymers that are marginal film formers. The term "marginal film formers" for purposes of this invention is herein used to describe a polymer existing at a temperature T which is generally within or about the Tg point of the polymer and which is intermediate a good film former and a non-film former in its flow characteristics. It is, of course, understood that the cutoff point between a non-film former and a marginal film former on the one hand and a marginal film former and a good film former on the other hand is not a specific point, but rather is a graduation or range of temperatures within which different amounts of polymer flow are occurring in an attempt to form a film.
For the purposes of this invention and in order to conveniently describe the ability of any particular polymer to attempt to form a film, the term "flow characteristics of a polymer" will be hereinafter used. This term may thus be defined as describing those characteristics of a polymer or polymeric material in a latex which tend to form the material into a coalesced mass or film.